Mark MacKinnon, who designs Mr McCain's advertisements, said he could not face being part of a campaign that "would inevitably be attacking" Mr Obama.

"I have met Barack Obama. I have read his book. I like him a great deal. I disagree with him on very fundamental issues but it would be uncomfortable for me and it would be bad for the McCain campaign," he told National Public Radio.

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He said he would support his current boss, but would do so "from the sidelines". Given the Illinois senator's new status as the narrow Democrat frontrunner, Mr MacKinnon could be out of a job within a few months.

The confession by Mr MacKinnon, who was a Democrat before joining George W Bush's presidential campaigns, underscores the respect and affection that many moderate Republicans and independents feel for Mr Obama as he attempts to become the first black US president.

Lincoln Chafee, the former Republican senator for Rhode Island, endorsed Mr Obama, despite the fact he was unseated in 2006 partly because Mr Obama campaigned hard for his opponent.

Mr McCain, who is likely to seal the Republican nomination within weeks, will face a stiff challenge for independent voters who have been a traditional strength. At a rally in Georgia recently a senior McCain organizer said as a Roman Catholic he remembered his pride when John F Kennedy became the first Catholic president.

"An Obama presidency would say to African Americans 'you are finally part of the club'. It would be a wonderful thing," he said, asking to remain nameless. Mr Obama has dubbed Republicans leaning his way "Obamacans".

Many are in awe of his oratorical powers, others respect his early opposition to the Iraq war or his appeal across party lines. His message of unity and common purpose has so far proved a powerful draw in primary elections among swing voters, helping him pull ahead of Hillary Clinton in the race for delegates who will ultimately decide the nomination.

Since Super Tuesday on Feb 5, the 46-year-old first-term senator has won eight contests in a row, leaving his rival dependent on strong showings in Texas and Ohio on Mar 4.

Mrs Clinton's campaign has been in flux, with the campaign manager and her deputy stepping down, and reports of infighting between senior staff. She is struggling to keep up with Mr Obama's grassroots and online fundraising network.

She was however boosted by polls showing she held substantial leads over Mr Obama in Ohio and Pennsylvania, which votes in April. Both states have large blue collar populations that have so far proved reliable support for the New York senator.

Indicating a more aggressive approach in the coming weeks, Mrs Clinton released her first negative ad, criticising Mr Obama for failing to accept an invitation to debate in Wisconsin, which holds its primary on Tuesday. "Maybe he'd prefer to give speeches than have to answer questions," the announcer says.